Jiang, Xiuyun; (2019) Computational Studies of Electron Transfer in Multi-Heme Proteins. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Multi-heme proteins are fascinating biomolecules that bind several redox-active heme cofactors in close distance to shuttle electrons across the bacterial membrane. Yet, the kinetics and time scales on which these electron transfer (ET) events occur is not well known and difficult to probe experimentally. The central aim of this thesis is to compute and to quantify heme-heme ET rate constants and electron flux through solvated multi-heme proteins. To this end, density functional theory and molecular dynamics simulation are deployed to compute heme-heme ET parameters in the framework of (non-adiabatic) Marcus theory, the central theory underlying such ET events. Three ubiquitous multi-heme proteins have been studied, which bind 4 and 10 heme cofactors. Our calculations revealed that electron transfer through these proteins is strongly enhanced by cysteine side chains that are inserted in the space between heme groups. We believe this to be a general design principle in this family of proteins for acceleration of ET steps that would otherwise be too slow for biological respiration. Our computational protocol has been verified via comparing our predicted time scale of heme-heme ET with the corresponding ET rate constant measured from pump-probe spectroscopy. The maximum, protein-limited electron flux is ≈ 10^5 - 10^6 s^-1. Such efficiency in long-range electron transfer indicates that multi-heme proteins are promising candidates for biological nano-electronic devices.

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